Well, I'm somewhat embarrassed to admit that this very lengthy thread has turned out to be mostly a false alarm - I was misled by the diagnostic method I was using. Following the advice of more than one person in this thread, I bought a VOM that allows current measurements of up to 10 amps, and connected its leads between the negative battery terminal and the cable that was attached to it. Initially, it read 3.6 amps - pretty close to the ~4.2 amps I was reading using the voltage-drop-across-the-1-ohm-resistor method. But after just a few seconds, I noticed it had dropped to 3.15 amps. I waited about 10 minutes (didn't get the exact time), and lo and behold, the current reading plummeted all the way down to about 35 mA, meaning everything appears to go completely to sleep, and the current draw is perfectly normal.

So, I had the battery, alternator, and starter all tested - all tested perfect. So, the question remains as to why the battery goes dead if the X5 isn't driven for a few days. The only thing I can come up with is, possibly the initial current drain of nearly 4 amps is unusually high. That, combined with the fact that the vehicle is used mostly for short trips, means that the 4 amp draw can happen many times, without the alternator (which delivers 49 amps at idle) getting the chance to adequately charge the battery. And it doesn't even have to be driven on a short trip to experience the initial 4 amp draw - I would assume that just unlocking the doors "wakes up" the systems, triggering the draw.

Anyway, I don't have any better theories. Does anyone have any idea as to what a normal initial, "pre-sleep" current draw should be? I've measured it on some of our other cars, and never saw anything close to what the X5 is producing. Thanks; sorry for wasting everybody's time, and I hope my experience provides something others can learn from - I've definitely learned a lot.

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2003 X5 4.4i
1992 Porsche 968
2007 328i
2014 Subaru Forester XT